In an optical communication system, plural optical signals with different wavelengths, but belonging to the same wavelength-band, such as the 1.3 .mu.m-band or 1.5 .mu.m-band, are multiplexed and transmitted through an optical fiber, in order to remarkably improve efficiency of transmission. According to this method of transmission, information of multi-channels can be transmitted through a single optical fiber. An optical communication system in which optical signals are multiplexed in the narrow band of wavelength as mentioned above, is called a narrow-band WDM optical transmission system.
The narrow band WDM optical transmission system is now universally admitted as the most promising system to meet increasing requirements for transmitting extensive information with extremely high bit-rates.
However, in the above mentioned optical transmission system, the spacing of the wavelengths of the adjacent optical signals is so narrow that it is very difficult to prevent interference from the adjacent optical signals, because the adjacent optical signals cannot be satisfactorily attenuated by optical filters at present. For example, if an optical signal is to be received is attenuated, there arises possibility that the adjacent optical signals are received, discriminated, and regenerated by a circuit provided for the optical signal to be received.